Do we love Johnny Depp as Captain Jack Sparrow? Of course we do. He worked up something strange and lovely for this character in the first Pirates of the Caribbean three years ago, and heís deliriously at it again in the new sequel, subtitled Dead Manís Chest. It is not just a matter of his eye makeup or his funny way of walking, running or (sometimes) sitting still  as when he discovers, to his dismay, that the cannibals have decided to make him the centerpiece of their banquet. It goes deeper than that: Jack is a modernist, unaccountably obliged to the mindless heroics not only of an antique movie genre, but to the whole ethos of an era when everyone heedlessly advances into action, swords drawn, instead of, more sensibly, retreating into their studies to think things over when danger threatens.

Jack, frankly, is an anachronism. A lot of his dialogue could be comfortably fitted into a contemporary Owen Wilson romantic farce, and a lot of his more muscular activities are desperate existential improvisations. Depp lets us see his mental gears whirring (and very often clanking) before he takes action that in some way subverts everyone's expectations. You might say that heís the anti-Errol Flynn.

The context of both Pirates movies aims for similar subversions. You will recall that in the first film the bad pirates that abducted the governor's daughter (Kiera Knightly) turned out to be representatives of the undead, which involved her swain (Orlando Bloom) and Jack with a lot of special-effects figures (they turned into skeletons when night fell). This was, I thought, a drag, but it was rendered tolerable by the wit and originality of Depp's performance. This time, the plot device is quite similar: The eponymous chest contains something that will return Davy Jones (Bill Nighy), king of the underseas underworld and, yes, captain of the The Flying Dutchman, no less, to the land of the fully living.

But Davy is half octopus, half man, and his crew seems to have been recruited from the Star Wars cantina of yore, though they have even worse skin problems and are covered with the kind of slime that has recently become fashionable among the villains in fantasy films. They are not very menacing or scary and neither are the threats they pose to Jack and his pals. There's also a whole thing with a giant squid that may put such ancients who attend the senior matinees in mind of Cecil B. DeMille's Reap the Wild Wind (1942). And that says nothing of the water wheel that comes loose from its mooring at an old mill and rumbles across the countryside, quite like the ferris wheel that rolled out of an amusement park in Steven Spielberg's 1941 to similarly mirthless effect.

These are ripoffs, not homages, but they are also emblematic of a movie that is essentially a special-effects extravaganza . It is, I think, a universal truth of movie making that effects are never funny. They can sometimes wow you, but they can't make you laugh, and Depp cannot stand up to the hubbub they create. No actor can. He can only serve them, which involves him in derring-do that any actor could do about as well as he can. He needs to be involved with us, not with the lunking machinery of the movie. When, for example, heís miming alarm and confusion he gets to do some terrific, ephemeral things with his eyes. Sometimes they are bright with half-formed schemes. Sometimes they are hard with a resolve that has not quite coalesced into a plan of action. Sometimes they are addled with a flickering panic he can't entirely hide. In these moments he takes us behind the conventional hero's stoic mask and allows us to see Jack as case of arrested development , a pre-sexual child pretending to be a man of decisive action and romantic elan. Mostly, though, he's forced to give a bits-and-pieces, fits-and-starts performance, fighting for his life against crowds of the undead and the unfunny.

Screen comedy is at its best when it pitches it tent close to the poverty line. Think Chaplin, who once said that all he needed to be funny was a park, a policeman and a pretty girl. Think Keaton, who once did a brilliant special-effects comedy, (Sherlock, Jr.) , where you were almost unaware of his very subtle camera tricks. Think Grant, Hepburn and their wayward leopard. For that matter, think Something About Mary, which pretty much took place in a cramped apartment. The minute the effects budget swells, it starts to crush the life out of comedy, which needs empty spaces to roam and some quality alone with the audiences in order to enlist its complicity in its subversions.

Well, you say, maybe Pirates of the Caribbean: Dead Manís Chest wasn't supposed to be all that funny. Maybe the fact that Depp decided to be brilliant was an unintended blessing bestowed on the original production, which the sequel is sort of stuck with. He was so good, doing, as he confessed, his imitation of the piratical Keith Richards, that he grabbed the reviews that brought in, as an unexpected increment, a crowd of grown-ups looking for some wit in an unlikely place  the multiplexes in summertime. This new film lends a certain credence to that supposition. It is as endless (at two and a half hours) as the Peter Jackson King Kong, and like that misbegotten film it is intended for the last mass audience left to the movies  adolescent males geekily parsing the effectiveness of its special effects. They will doubtless think theyíre swell. The director, Gore Verbinski, has spared no expense in realizing them, and there's no doubt in my mind that as a commercial proposition the film will be a winner. In every other way  as adventure yarn or as satire on that form or merely as an enjoyable entertainment featuring a wonderfully sly and subtle actor  it is not merely a loser. It is a disaster.